Does nursing really affect your kid's IQ?

The Breast and the Brightest

Does nursing really affect your kid's IQ?

In my corner of the world, breast-feeding is not really optional. Among the women with whom I talk about babies and kids, I can't think of anyone I know in my approximate age bracket who didn't try. My friends who quit at three months seemed like rebels. And when I cut off my sons, after more than a year each, I felt a little heartless because I know so many kids who zealously nursed into toddlerhood.

Nursing is cozy and nurturing, not to mention remarkably efficient—never again will I provide for my children's needs so gracefully (OK, that's not the right word for the pumping part of it). But it can also get a little fanatical. Mothers who adopt children are left out. And when breast-feeding doesn't come easily—an unusual but real occurrence—women sometimes go to great and uncomfortable lengths involving all manner of awkward contraptions. They do this because they think that they'll be depriving their babies if they give up as nursing failures. Breast-feeding is supposed to protect against childhood cancer, obesity, allergies, infections, and global warming (I made up only the last one). Some of the claims, like a lower rate of infant respiratory infections, seem to hold up; others, like reduced odds of adult obesity, probably don't.

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Now there's new evidence about the gold ring of breast-feeding benefits—extra IQ points. It's a finding with a twist. The researchers report that breast-fed babies get an average IQ advantage of 6.8 points—a nice step up—but only if they carry a certain genetic variant. If you've got the gene and your mother nurses you, she is making you smarter. If you don't have the gene, the nursing is for naught, IQ-wise. What are we to make of this?

Practically speaking, probably nothing. A series of caveats apply. This is only one study, and there are lots of other reasons to breastfeed (or not to). Plus, 90 percent of the population has the genetic variant that conveys the IQ boost, so the odds are in the suckler's favor. But as food for thought, this study has all kinds of goodies. It's a pretty riveting example of a dynamic that scientists call "G × E," for genes times environment—the notion that it's not nature or nurture that exclusively makes people who they are, but nature interacting with nurture. G × E starts cropping up a lot, once you look for it, and the concept makes sense intuitively. Why wouldn't the meshing of our genes and our experiences have its own influence? But G × E doesn't really resolve the hard questions about the haves and have-nots that the nature-nurture debate spawns. It merely reframes them—and opens the door to predict-the-future-of-your-kid information, via genetic testing, that for many parents is equal parts tantalizing and scary.

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The new breast-feeding-IQ study was conducted by the lab of Avshalom Caspi and Terrie Moffitt, smart and careful scientists whom I've written about before. Caspi and Moffitt looked at two large groups of kids, one of more than 1,000 from New Zealand and the other of about 2,200 from England. They asked mothers about whether they'd breast-fed via questionnaire, when the kids were 3 and 2 years old (not the best method for data-collection, but at least the kids were still young). In New Zealand, 57 percent of the kids were breast-fed; in England, 48 percent. Then the authors looked at a gene called FADS2. It plays a role in regulating the production of some special fatty acids present in breast milk, which may help spark cognitive development (though there is controversy about this, Slate contributor Sydney Spiesel tells me). There are three variants of this gene. Two of them include an allele (the "C allele") that conveys a significant IQ boost—but only in conjunction with breast-feeding. In other words, if you have the gene but your mother doesn't nurse you, you're a bit dumber than you otherwise would be. If you don't have the lucky allele, you don't get the IQ boost however much mother's milk you imbibe.

Previous studies have also linked breast-feeding to higher IQ, but they generally haven't ruled out the fact that breast-fed kids are also more likely to come from wealthier and better-educated families than formula-fed babies. What has looked like a correlation between nursing and smarts might really have been explained by other more predictable factors. Caspi and Moffitt, however, controlled for the confounding factors of social class and maternal IQ. And they still found a pattern of higher IQs in the breast-fed babies with the C alleles. Nor did the advantage seem related to the mothers' genotypes.

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Caspi and Moffitt point out that in the earlier annals of human history, when everyone was breast-fed, "genetic variation in FADS2 could have influenced individual differences in intelligence." In this sense, they say, "It is reasonable to ask whether FADS2 is a 'gene for' IQ." Maybe that helps explain why 90 percent of the population has the C allele—if it promotes intelligence, then it should win out in the process of natural selection. From a pro-breast-feeding point of view, this is all kind of lovely. Some babies have a gene that boosts intelligence, but only when it's activated by the delivery of mother's milk, and so, over time, more babies have the gene. Nature and nurture working in tandem, and to a good end.

OK, enough with the cave folk. What about us? Breast-feeding helps make most kids smarter, if the results of this study bear out in future research. In a birth order study that came out earlier this year, much was made of a three-point IQ edge for firstborns, so the nearly seven-point jump associated with breast-feeding and the C allele looks substantial. But the nursing doesn't help all kids—and so it's actually one more source of heightened inequity. Not just in the sense that kids who aren't breast-fed don't get the boost, a "shame-on-mothers" argument we're used to. No, now we have to add on a genetically based inequity. Of course, life is full of those, but they're still discomfiting, and perhaps all the more so in the presence of this sort of G × E interaction. There are two ways to lose out, genetic and experience-based. And to win, you need both to go your way. (We're talking about averages over populations, not one-to-one correlations, of course, but you can still see the point.)

Then there's the question of genetic testing. You could test a child for the C allele, and if she has it, feel even better about nursing her. You'd also feel worse if you can't. And if your baby is in the 10 percent without the lucky gene variant? You can still nurse, of course, but your rationale would be different—and wouldn't you feel ripped off by nature's grand lottery, and a little bitter about it, at least on a sore-nipple day?

This is the kind of information we may not really want to have. The sinisterish ramifications are even starker in other research that Caspi and Moffitt have done, in which they've found a G × E relationship for a gene that's linked to depression in connection with an experience of serious trauma, like child abuse. Do we want to know who has this predisposition? (A test already exists.) If you were thinking about adopting a child who might have had some hard experiences, wouldn't you choose the child with the protective variant of the gene over the one without it? And what does that mean for the genetically unblessed kid? If you ran an adoption agency, would you give kids the test? Questions for the near future, or, really, for now.